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The reason is that the risks associated with the obstacle are greater than the risks associated with the incursion.

Just an asterisk to your argument here: the risk of a vehicle on the trail are huge: you could have someone kill lots of people. BUT the likelihood of that scenario is very low, based on experience. In the end, we should take into consideration both the potential harm and the likelihood of the event coming to pass when making policy.

But I think that all supports you point: With bollards (of any kind), the potential harm is broken bones, and the likelihood is moderate. With vehicles on the trail, the potential harm is death, but the likelihood is very low.

Just an asterisk to your argument here: the risk of a vehicle on the trail are huge: you could have someone kill lots of people. BUT the likelihood of that scenario is very low, based on experience. In the end, we should take into consideration both the potential harm and the likelihood of the event coming to pass when making policy.

But I think that all supports your point: With bollards (of any kind), the potential harm is broken bones, and the likelihood is moderate. With vehicles on the trail, the potential harm is death, but the likelihood is very low.

Thanks. The other problem is that we know not where the crazy/oblivious driver will attempt to enter. So it's easy to say, "a bollard there may have stopped her." However, the logical endpoint of that reasoning is that we have to put them everywhere. So it's not one bollard v. one car. It's thousands of bollards v. one or a few cars.

We also don't know what is going on in someone's head. A determined driver will find a way to get on the trail. Many places I've seen bollards used they can just be driven around.
And if not, then they tend to be some sort of fortress like at the White House.

The lady in question who, thankfully, did not injure our friend, Sunyata, may have been deterred by better design, like the new design at the Vienna Community Center. Or not.

FHWA has essentially done the calculation you describe, which is why they recommend against their use except as a last resort after other strategies have tried and failed.